1. Field of the Invention
The present invention relates to a system control device having a self-protection function, and more particularly to a system control device that is capable of steadily maintaining a controlled state.
2. Background Art
In general, a system control device containing a power semiconductor or the like provides feedback control. More specifically, such a system control device exercises control in accordance with a control command value that is defined to reduce the deviation between an actual control value and a target control value, which is to be attained by the system control device. The control command value, which is to be determined for feedback control, generally contains an integral term. A booster device and an electric power steering device are disclosed, for instance, in JP-A-2006-62515 as a system control device that provides feedback control.
Further, the system control device generally incorporates a self-protection function for the purpose of protecting itself, for instance, against short-circuiting, temperature extremes, and high voltage. The self-protection function is exercised to provide self-protective control by temporarily stopping a control operation or by providing control at an output level lower than a target control value. As regards the booster device and electric power steering device disclosed in JP-A-2006-62515, a soft start configuration for inhibiting an output voltage overshoot at the beginning of control is described as the self-protection function.
A system control device that incorporates a self-protection function and provides feedback control can operate in accordance with a target control value or a control value close to the target control value. Further, such a system control device can prevent itself from being adversely affected by exercising the self-protection function to stop itself or operate at a low output level. However, there is the following problem with a system control device that incorporates both feedback control and self-protection functions. More specifically, there is a great discrepancy between a target control value and an actual control value during a self-protection period. Therefore, the integral term targeted for feedback has an undue value. If the integral term is excessive during the self-protection period, the control command value is computed in accordance with the excessive integral term when a self-protection process is terminated to exercise control again in accordance with the target control value. Consequently, feedback control is exercised through the use of an undue control command value that is greater than the control command value required to attain the target control value. This causes a problem such an overshoot in which voltage or current is excessive.